Articles of aluminum or an aluminum alloy are typically subjected to an anodic oxidation process to improve the hardness and corrosion resistance of the surface of the article. As used herein, the term aluminum includes pure or substantially pure aluminum as well as alloys of aluminum containing, in general, at least about 50% by weight of aluminum. Examples of other metals which may be present in such aluminum alloys are silicon, bismuth, copper, nickel, zinc, chromium, lead, iron, titanium, manganese, and the like. The anodic oxidation of an aluminum surface results in the formation of a uniform, translucent, highly porous aluminum oxide film. The anodic oxide film can also serve an aesthetic or decorative function and may be clear, inorganically dyed, or electrolytically colored.
Generally, these anodic aluminum oxide films result from the passage of a direct electric current through an acidic electrolyte solution at temperatures ranging from 32.degree. F. to 90.degree. F. Typically, the acidic electrolyte solution will employ sulfuric acid at concentrations from 140 to 200 grams per liter g/L.
Anodic oxidation of aluminum is intended to provide a protective coating or film of aluminum oxide on the aluminum surface. Although this anodic oxide layer is more resistant to corrosion than the untreated aluminum surface, the porous structure of the aluminum oxide layer ultimately renders it vulnerable to corrosion and degradation, particularly to that caused by external chemical agents. As a result, anodized aluminum oxide films or surfaces are commonly subjected to a process known as sealing. It is currently theorized that the sealing process closes or fills the pores via hydration and/or precipation of one or more compounds. Aluminum articles treated in such a manner are generally usable in a wide variety of end use applications, even those employing severe environmental conditions.
High quality sealed anodic aluminum oxide films should exhibit superior resistance to corrosion and degradation caused by external chemical agents.
Prior art sealing processes can generally be divided into three categories: (a) hydrothermal sealing processes, (b) mid-temperature sealing processes, and (c) low temperature sealing processes.
Hydrothermal sealing processes employ steam or boiling water to seal the anodic oxide coating. This process involves the hydration of the oxide coating which results in the constriction of the surface pores. Although good quality sealed films are generally obtained, the disadvantage of this process is the extremely high energy cost associated with its operation.
Mid-temperature sealing processes operate at temperatures between 170.degree. to 200.degree. F. They generally employ aqueous solutions of heavy metal salts such as nickel or cobalt. Mid-temperature sealing processes enjoy a significant savings in energy consumption as compared to hydrothermal sealing processes, but present significant waste disposal problems in view of the presence of heavy metals. Such solutions often require expensive pretreatments prior to disposal.
Low temperature sealing processes also suffer from the presence of heavy metals. Such processes typically employ nickel salts such as nickel fluoride and operate at temperatures of about 90.degree. F. In addition to the waste disposal problems, low temperature sealing processes suffer from the disadvantage of producing a sealed anodic film having a very low crazing temperature.
Thus, the prior art has failed to provide a process or composition for use in sealing anodized aluminum oxide films which provide the economic advantages of the mid-temperature and low temperature sealing processes but retain the overall film quality and waste disposal advantages of the hydrothermal sealing processes. A review of representative prior art patents addressing this problem illustrates the lack of a satisfactory solution.
For example, U.S. Pat. No. 4,939,001 to Brodalla et al. discloses the use of sealing solutions containing phosphonic acid and the alkali metal salts and alkanolamine salts of those acids. It also discloses the use of alkali metal acetates and alkali metal hydroxides as buffering agents. Although this sealing composition does not appear to contain heavy metals, it is disadvantageous because careful control of the phosphonic acid concentrations are required. At increased concentrations, phosphorus containing compounds inhibit sealing. Also, as disclosed by the working examples, high application temperatures above 200.degree. F., (i.e., from 205.degree. to 208.degree. F.), are required for desirable surface characteristics. Thus, the disclosed process and composition are disadvantageous due to operating costs and quality control requirements.
British Patent No. GB 2,254,622 A discloses an aqueous sealant composition comprising at least one alkaline earth metal salt and a particular anti-smut compound. Per the teachings of this patent application, both compounds are required to achieve high quality sealed surfaces.
Prior art sealing processes employing the use of alkali metal silicates are particularly disadvantageous because such alkali metal silicate compositions must be used at low concentrations. At concentrations greater than about 0.05 g/L, such silicates can precipitate out and render the sealing composition unusable. In addition, it has been found that the soluable silicates can inhibit the sealing process itself.
Thus, it is an object of the invention to provide a process and composition for sealing anodized aluminum articles or surfaces which has relatively low energy costs, is environmentally friendly and provides a sealed anodic aluminum oxide film of high quality.
It is a further object of the invention to provide a process and composition for providing high quality sealed aluminum oxide films which employs an effective amount of a source of lithium ions.
Finally, it is a further object of the invention to provide a process and composition for sealing anodized aluminum oxide films which employs an effective amount of a source of alkali metal ions, wherein the aqueous sealing solution is substantially free of compounds containing an element selected from the group consisting of the heavy metals, silicon, and phosphorus.